And yet Republicans are worried about their chances in Senate races they aren't normally concerned about.

Take Tennessee, for example. President Trump will go to Nashville on Tuesday night to campaign for Rep. Marsha Blackburn (R-Tenn.), who's trying to replace retiring Sen. Bob Corker (R). Tennessee is a Trump state, but Blackburn is expected to face a race against former popular Democratic governor Phil Bredesen that's more competitive than normal.

Not helping Republicans: They are skittish after losing a Senate seat in Alabama last year.

Tennessee is just one of four races in which Democrats have a chance of flipping a Republican-held seat and, thus, a chance of ending Republicans' majority. Here's a rundown on all four, ranked in order of least to most likely:

4. Texas: No, “Texas” isn't a typo. As I wrote in April, there are three campaign dynamics going Democratic Rep. Beto O'Rourke's way in his uphill battle to unseat Sen. Ted Cruz (R):

The buzz: Liberal activists were intrigued by O'Rourke's social media savvy and the fact that he gave up a comfy El Paso-area congressional seat to challenge a guy who came close to winning the Republican nomination for president.

The fundraising: O'Rourke raised $6.7 million through the first three months of 2018, an insane number for any Senate candidate anywhere, let alone a Democrat in Texas. Cruz said he raised less than half of that in the same period.

But: Unseating a Texas senator is one of the hardest things to do in politics. No Democrat has won a statewide race in Texas in 24 years, which the Associated Press says is the nation's longest losing streak.

3. Tennessee: If Republicans lose the Senate in November, writes The Fix's Aaron Blake, Tennessee may be the state that does them in. Corker has said he won't campaign against the likely Democratic nominee, Bredesen, going so far as to call him a “very good governor.” Two recent polls show Bredesen has a lead over Blackburn.

But: Did I mention Tennessee is a Trump state? Trump won it by 26 points. So maybe Blackburn doesn't really want Corker's help, given that he's got an on-again, off-again relationship with the president that at its nadir had him accusing Trump of being capable of starting World War III.

2. Arizona: This race is complicated, because Arizona is shaping up to be ground zero for Republicans' intraparty war. Also, there might not be just one seat for Republicans to defend.

Trump critic Sen. Jeff Flake (R) is retiring at the end of the year, and Democrats have made this seat competitive by recruiting moderate, even GOP-friendly Rep. Kyrsten Sinema. Meanwhile, Republicans are mired in an unpredictable primary in which two candidates on the far right, Kelli Ward and former sheriff Joe Arpaio, could spoil more moderate GOP Rep. Martha McSally's chances to make it to the general election.

Rep. Martha McSally (R-Ariz.) waits to speak at a rally January in Phoenix. (Matt York/AP)

Then there's Sen. John McCain's (R-Ariz.) seat. The 81-year-old technically has four more years left in what he has said will be his last term. But he's been back home in Arizona all year battling a deadly form of brain cancer, and it's an open question whether he'll stay in office or for how much longer. If his seat does become open this year (a big if, on which we'll have more clarity this week), Republicans will have to split their efforts to defend two open seats in a state that Trump won by just three points.

But: Democrats have thought they were competitive in Arizona before only to lose. McCain easily won his Senate seat in 2016 after awkwardly balancing his on-again, off-again relationship with Trump.

1. Nevada: Nevada is Democrats' best pickup opportunity because it's also one of the most competitive Senate races anywhere in the country. And that's because Sen. Dean Heller (R) is one of the most vulnerable senators of either party. He's the only Republican senator running for reelection in a state Hillary Clinton won, and Democrats think Heller has upset his base too much by voting against a bill to repeal the Affordable Care Act.

Heller and Trump at the White House in July. (Jabin Botsford/The Washington Post)

As we said earlier, Democrats' attention will be divided, playing whack-a-mole with at least eight competitive Senate races where they are the ones on defense.

That means the top pickup opportunity for Democrats is neither a given, nor is Democrats' chance to flip the chamber. But Democrats' path to taking back control of the Senate goes through these four states.